test(package): demonstrate target dir not excluded from backups

  Add a test that documents the current behavior where `cargo package`
  creates the target/package directory without excluding it from backups.
  The CACHEDIR.TAG file does not currently exist in the target directory,
  causing backup tools to include these temporary build artifacts.

  This test currently passes by asserting the buggy behavior (CACHEDIR.TAG
  does not exist). The next commit will fix the issue and update the test
  to verify correct behavior.

Author: SepehrGanji

tests/testsuite/package.rs

@@ -7807,3 +7807,34 @@ fn publish_to_alt_registry_warns() {
 "#]])
         .run();
 }
+
+#[cargo_test]
+fn package_dir_not_excluded_from_backups() {
+    // This test documents the current behavior where target is NOT excluded from backups.
+    // After the fix, this test will be updated to verify that CACHEDIR.TAG exists.
+    let p = project()
+        .file("src/main.rs", r#"fn main() { println!("hello"); }"#)
+        .build();
+
+    p.cargo("package --allow-dirty")
+        .with_stderr_data(str![[r#"
+[WARNING] manifest has no description, license, license-file, documentation, homepage or repository
+  |
+  = [NOTE] see https://doc.rust-lang.org/cargo/reference/manifest.html#package-metadata for more info
+[PACKAGING] foo v0.0.1 ([ROOT]/foo)
+[PACKAGED] 4 files, [FILE_SIZE]B ([FILE_SIZE]B compressed)
+[VERIFYING] foo v0.0.1 ([ROOT]/foo)
+[COMPILING] foo v0.0.1 ([ROOT]/foo/target/package/foo-0.0.1)
+[FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
+
+"#]])
+        .run();
+
+    // Currently, CACHEDIR.TAG does NOT exist in target/
+    let cachedir_tag = p.root().join("target/CACHEDIR.TAG");
+    assert!(
+        !cachedir_tag.exists(),
+        "CACHEDIR.TAG should not exist (documenting current buggy behavior)"
+    );
+}
+